In industrial plants, piping systems for boilers, steam lines, liquid chemicals and the like are used to transport the materials from one location to another. The materials can be highly corrosive to the piping or can be transported under high temperatures and pressures which will cause extensive wear to the piping. Failure of the piping systems can be catastrophic in many instances, both in terms of environmental concerns and in terms of unscheduled emergency down time for a plant.
Accordingly, systems have been developed to record the history of wear on the piping. A hand-held ultra-sonic thickness gauge has been developed for on-site data logging. The ultra-sonic gauge will measure the thickness of a pipe wall and record the measurement data in a memory together with other relevant information such as temperature, reading location and conversion. The data is downloaded onto a personal computer which then processes the data into an effective maintenance schedule and history of the plant piping system. Such systems are available under the trademark DML DL by KRAUTKRAMER BRANSON.
In order to obtain an accurate history of measurements taken from the piping systems, the measurements must be taken from the same location so that wear from corrosion or erosion can be identified. If the measurements are not taken from the same location, false readings can be obtained which may result in accelerated repair schedules.
The most common method of taking same location measurements is to place a grid pattern on the pipe component. The outside area of the pipe component is divided into a grid pattern. The size of the grid pattern can vary but many industries, such as nuclear utilities, have standardized the sizes. Grid sizes can vary from 1 to 6 inches (2.54 to 21.24 cm), depending on the size of the component which can vary from 2 to 30 inches (5.08 to 76.2 cm).
To commence an inspection, two persons normally will measure and draw the grid pattern on the component. A marker, paint, sticker or other marking device is used to scribe lines circumferentially around the component and axially along the length of the component. The spacing between all lines is maintained to as near the designated grid size as possible. Normally, it takes 6 to 20 person hours of labour to lay out and draw the grid lines.
Once the grid lines are in place, the inspection can take place. An ultra-sonic probe takes a thickness measurement where the lines intersect. The data is stored for later analysis by the computer.
A second method which is used is to place thin strips of a width of a corresponding grid size about the circumference of the component. The strips have a series of spaced holes 1/2 to 1/4 inch (1.27 to 0.635 cm) corresponding to the grid size used. Two persons place the strips about the component, attempting to maintain the grid size. As the strip is placed in the proper position, the hole positions are marked on the component using markers or spray paint. The strips are removed and then the readings using an ultra-sonic probe are taken and processed.
Both of these methods are prone to errors in the layout of the grid and also of the inspectors in taking the readings. The inspector is required to trace each grid line or painted dot back to a starting reference point to determine the identification of the inspection point being measured. This results in the consumption of considerable inspection time which results in data which is not very accurate when repeating the inspection for a determination of wall losses at a future date.
Further, it is quite common for the grid markings or painted dots to be missing for future inspections. Remarking is then required, which remarking is never in the exact location as the previous markings.